A conventional radar device transmits, as a radar wave, a transmission signal which is frequency-modulated with a triangle-shaped modulation signal, and whose frequency is repeatedly increased and decreased at fixed time intervals, as shown in FIG. 16. The radar device then receives a reflected wave generated by reflection of this radar wave from a target and generates a received signal, and also generates a beat signal by mixing the received signal with the transmission signal. The radar device then specifies the frequency of this beat signal (referred to as the beat frequency from here on) both for a sweep interval at the time of an up-chirp during which the frequency of the transmission signal increases, and for a sweep interval at the time of a down-chirp during which the frequency of the transmission signal decreases. The radar device then calculates the distance R to the target and the speed V relative to the target by using the following equations (1) and (2) (or the following equations (3) and (4)) on the basis of this specified beat frequency fu at the time of an up-chirp, and this specified beat frequency fd at the time of a down-chirp.
                              f          u                =                                                            2                ⁢                B                                            c                ⁢                                                                  ⁢                T                                      ⁢            R                    +                                                    f                0                            c                        ⁢            2            ⁢                                                  ⁢            V                                              (        1        )                                          f          d                =                                                            2                ⁢                B                                            c                ⁢                                                                  ⁢                T                                      ⁢            R                    -                                                    f                0                            c                        ⁢            2            ⁢            V                                              (        2        )                                R        =                                            c              ⁢                                                          ⁢              T                                      4              ⁢              B                                ⁢                      (                                          f                u                            +                              f                d                                      )                                              (        3        )                                V        =                              c                          4              ⁢                              f                0                                              ⁢                      (                                          f                u                            -                              f                d                                      )                                              (        4        )            where B shows the width of the frequency variation range of the transmission signal, f0 shows the center frequency of the transmission signal, T shows the time required for the modulation of one cycle, and c shows the speed of light. It is assumed that the modulation time T is short, and the distance R to the target and the speed V relative to the target do not vary between an up-chirp and a down-chirp.
As mentioned above, the conventional radar device can detect the distance to a target and the speed relative to the target by pairing the beat frequency at the time of an up-chirp with the beat frequency at the time of a down-chirp. However, an offset occurs between the beat frequency acquired at the time of an up-chirp and the beat frequency acquired at the time of a down-chirp even though they are the ones associated with an identical target. Particularly in an environment where a plurality of targets exist, i.e., a plurality of beat frequencies exist, it is necessary to determine which beat frequency at the time of an up-chirp matches that at the time of a down-chirp, but it is very difficult to carry out the determination.
The above-mentioned problem will be explained by using FIGS. 17 and 18. For example, it is assumed that a target is detected both for an up-chirp and for a down-chirp, as shown in FIG. 18, by using a transmission pattern in which an up-chirp and a down-chirp are repeated for each scan, as shown in FIG. 17. In FIG. 18, a mark □ shows a target 1, a mark ∘ shows a target 2, and a mark x shows an unnecessary signal, and it is assumed that the target 2 is not detected at the time of transmitting a down-chirp. Because an offset due to a target speed occurs between an up-chirp and a down-chirp, a plurality of combinations are generated. Therefore, an unnecessary target occurs and there is also an undetectable target like the target 2. The following methods are disclosed as a measure taken against this problem (for example, refer to patent reference 1 to 4).
The patent reference 1 discloses a method of arranging beat frequencies acquired according to sweep period in ascending order of beat frequency, and pairing the beat frequency at the time of an up-chirp with the beat frequency at the time of a down-chirp in such a way that the arrangement order is maintained. As a result, an environment including two or more targets can be supported. However, when a false alarm occurs either at the time of transmitting an up-chirp or at the time of transmitting a down-chirp, and there exists a beat frequency caused by an unnecessary signal component other than signals associated with a target in the case of using this method, i.e., when target detection conditions differ between at the time of transmitting an up-chirp and at the time of transmitting a down-chirp and the number of beat frequencies at the time of transmitting an up-chirp is not the same as that at the time of transmitting a down-chirp, there is a high possibility that pairing between beat frequencies (simply referred to pairing from here on) is not carried out correctly.
Further, patent reference 2 discloses a method of predicting the current distance to a target and the current speed relative to the target on the basis of the distance to the target and the speed relative to the target which were detected in the past, and calculates a beat frequency at the time of transmitting an up-chirp and a beat frequency at the time of transmitting a down-chirp on the basis of this predicted values. Then, a detected pair having frequencies close to the pair of these predicted beat frequencies is determined as a true pair. Further, patent reference 3 discloses a method of converting a tracking predicted value into a predicted beat frequency after tracking a target, pairing the predicted beat frequency with an inputted beat frequency, and updating the tracking estimated value. However, a problem with these methods is that when a target detected in the past or a target for tracking is an erroneous target, the erroneous target remains continuously and this causes errors in the subsequent target detection.
On the other hand, patent reference 4 discloses a method of, as shown in FIG. 19, directly tracking the beat frequency acquired at the time of transmitting an up-chirp and the beat frequency acquired at the time of transmitting a down-chirp, and calculating the distance to the target and the speed relative to the target by using beat frequency time series data having a correlation in a temporal direction. However, a problem with this method is that the distance and the speed are calculated based on the premise that the speed does not vary between scans (the speed is constant), the accuracy of estimating the distance and the accuracy of estimating the speed degrade on conditions that the premise is not established.